To protect regional and global environment, exhaust-gas-cleaning catalyst carriers and particulate-matter-capturing filters comprising ceramic honeycomb structures are used to reduce harmful substances contained in exhaust gases discharged from automobile engines.
As shown in FIG. 2, a conventional ceramic honeycomb structure 20 comprises perpendicularly crossing cell walls 23 forming large numbers of flow paths 24 and an outer peripheral wall 21, and has a cross section perpendicular to its flow paths usually in substantially circular or elliptical shape. The outer peripheral wall 21 of the ceramic honeycomb structure 20 is held by grip members (not shown) formed by a metal mesh or a ceramic mat, etc. in a metal container (not shown), such that the ceramic honeycomb structure 20 does not move during operation.
The ceramic honeycomb structure 20 is produced by the steps of (1) blending starting materials such as a ceramic material (for instance, cordierite powder), a molding aid, a pore-forming material, etc. with water to prepare a moldable ceramic material, (2) extruding the moldable ceramic material through a honeycomb-shaped die to form a green body having a honeycomb structure integrally comprising cell walls 23 and an outer peripheral wall 21, and (3) drying and sintering the green body. With such steps, a ceramic honeycomb structure 20 having a predetermined shape and strength with fine pores in cell walls 23 is obtained.
For filters for cleaning exhaust gases discharged from diesel engines, a large ceramic honeycomb structure 20 as shown in FIG. 2, which has an outer diameter D of 150 mm or more and a length L of 150 mm or more with cell walls 23 as thin as 0.2 mm or less, may be used. In the production of such a large ceramic honeycomb structure 20 with thin cell walls, a green ceramic honeycomb body obtained by the extrusion of a moldable ceramic material has such insufficient strength that part of cell walls 23 at edges of an outer peripheral wall 21 of the green body are deformed by its own weight. The sintering of the deformed green body fails to provide a ceramic honeycomb structure 20 having desired strength.
To solve this problem, as shown in FIGS. 1(a) and 1(b), JP 5-269388 A discloses a honeycomb structure 10 having an outer peripheral wall 12 fixed to a ceramic honeycomb body 11, the outer peripheral wall 12 being produced by filling grooves 15 formed by cells 14a on the outer peripheral surface among large numbers of cells 14 defined by cell walls 13 with a paste-like coating material 12c obtained by blending cordierite particles and/or ceramic fibers and colloidal oxides (colloidal silica, colloidal alumina, etc.) as main components with water, and drying or drying and sintering it. JP 5-269388 A describes a method for forming the outer peripheral wall 12 by leaving the applied coating material 12c to stand for 24 hours in the air, and drying it at 90° C. for 2 hours.
In the drying method described in JP 5-269388 A that heating is conducted from outside, a surface layer 12s of the coating material 12c is first heated, and heat is gradually conducted to the inside 12n. Accordingly, the surface layer 12s of the coating material 12c is first dried, and then water in the portion 12n moves toward the surface, and evaporates from the already drying surface layer 12s, so that drying proceeds in the inner portion 12n. As a result, difference in the water content occurs between the surface layer 12s and inside portion 12n of the coating material 12c during a drying process, resulting in difference in drying shrinkage, which makes the coating material surface 12s vulnerable to cracks 16. Particularly when the outer peripheral wall is thick, or when a heating temperature is elevated to shorten the drying time, there is a large water content difference, resulting in further cracks 16. If the outer peripheral wall had such cracks, cracks would be starting points of breakage, undesirably reducing the strength of the ceramic honeycomb structure and causing heat shock cracking.
JP 2006-298745 A discloses a ceramic honeycomb structure 10 coated with a coating material 12c as shown in FIGS. 1(a) and 1(b). This reference describes that an outer peripheral wall formed by a slurry comprising crushed porcelain having a particle size of 15-75 μm and 26-34% by mass of water as a coating material 12c is resistant to cracking even by forced drying (drying with far-infrared rays and/or hot air), resulting in reduced production time.
However, particularly when a large ceramic honeycomb structure having an outer diameter D of 150 mm or more and a length L of 150 mm or more for diesel engines, etc. is produced, forced drying using far-infrared rays and/or hot air would be difficult to uniformly dry the outer peripheral wall 12 even though a coating material comprising the crushed porcelain described in JP 2006-298745 A were used, resulting in partially uneven drying. As a result, the outer peripheral wall 12 is provided with densified portions and less densified portions, materials move in the coating material during a drying process. It makes it likely that low-strength portions of the outer peripheral wall are cracked by small shock during handling, and that part of the outer peripheral wall is peeled. Thus, the outer peripheral wall is not strongly held by grip members in a metal container, so that the ceramic honeycomb structure may be broken by moving during operation.
Although cracks in the outer peripheral wall 12 may be mended by filling a ceramic material, for instance, as described in JP 2005-144284 A, a large number of cracks need many mending steps, resulting in extremely lowered production efficiency. Further, when spherical colloidal silica as colloidal oxide is added to the coating material, the outer peripheral wall 12 is not well bonded to ceramic particles (aggregates) in the coating material, so that the dried outer peripheral wall fails to have high strength.